Exploring possibilities of band gap measurement with off-axis EELS in TEM

At a Glance

Metadata	Details
Publication Date	2018-03-29
Journal	Ultramicroscopy
Authors	Svetlana Korneychuk, B. Partoens, Giulio Guzzinati, Rajesh Ramaneti, Joff Derluyn
Institutions	University of Antwerp, IMEC
Citations	11
Analysis	Full AI Review Included

Technical Documentation and Analysis: Off-Axis EELS for Wide Band Gap Material Characterization

This document analyzes the research on off-axis Electron Energy Loss Spectroscopy (EELS) utilizing a Bessel aperture in a Transmission Electron Microscope (TEM) for precise band gap measurements in high-refractive-index semiconductors, with a focus on diamond materials.

Executive Summary

The reported research presents a highly effective technique for measuring fundamental electronic properties, such as the band gap, in wide band gap (WBG) materials like diamond, GaN, and AlN, while maintaining nanometer-scale spatial resolution.

Core Achievement: Successful measurement of band gaps in high-refractive-index materials (Diamond, n=2.4) using off-axis EELS in TEM/STEM mode.
Methodology: The use of a circular (Bessel) aperture enforces momentum transfer selection, fundamentally suppressing “parasitic” signal losses.
Noise Reduction: The technique achieves dramatic suppression of the Zero Loss Peak (ZLP) and Cherenkov radiation, improving the signal-to-noise ratio (SNR) by > 5 orders of magnitude compared to conventional methods.
Material Validation: Experimental measurements confirmed the direct band gap of diamond (NCD) at 7.4 eV, alongside validation of GaN (3.3 eV) and AlN (5.6 eV) band gap onsets.
High-Voltage Efficacy: The suppression mechanism remains highly effective even at challenging high acceleration voltages (up to 300 keV).
Spatial Resolution: The method facilitates high spatial resolution mapping (estimated to be in the few nm range) essential for characterizing multilayered heterostructures.

Technical Specifications

The following hard data points were extracted from the simulations and experimental results concerning the material properties and EELS performance parameters.

Parameter	Value	Unit	Context
Acceleration Voltage (High)	300	keV	Tested for suppression of Cherenkov losses
Acceleration Voltage (Low)	60	keV	Used for high-resolution spatial mapping
Diamond Film Thickness (Simulated)	50	nm	Plan-parallel geometry model
Diamond Refractive Index (n)	2.4	-	Characteristic of high-index materials
Diamond Direct Band Gap (E_bg)	7.4	eV	Measured experimental onset value
Diamond Indirect Band Gap (E_bg)	~5.5	eV	Theoretical onset value
GaN Band Gap Onset	3.3	eV	Measured experimental value in heterostructure
AlN Band Gap Onset	5.6	eV	Measured experimental value in heterostructure
ZLP Suppression (Bessel vs. Standard)	> 5	orders of magnitude	Achieved at 300 keV
Spatial Resolution	Few	nm	Estimated for mapping GaN/AlGaN/AlN layers
Bessel Aperture Material	1	µm	Gold (Au) film used for fabrication
Convergence Angle Tested (High)	8	mrad	Used for higher spatial resolution
Convergence Angle Tested (Low)	0.5	mrad	Used for maximizing momentum resolution

Key Methodologies

The study relies on manipulating electron beam transfer and collection geometry within the TEM/STEM to isolate low-loss inelastic scattering signals from unwanted parasitic effects.

Conical Illumination: The core method uses a conical illumination scheme achieved by inserting an annular aperture (Bessel aperture) in the condenser plane of the microscope.
Off-Axis Acquisition: The spectrometer entrance aperture is placed off-axis relative to the unscattered electron beam in the diffraction plane.
Momentum Transfer Selection (q Selection): This configuration enforces strict selection criteria on the perpendicular component (q_⊥) of the momentum transfer, effectively creating a “q-donut” of allowed transitions.
Parasitic Loss Suppression: The momentum transfer selection avoids the very low scattering angles (< 0.1 mrad) where Cherenkov radiation and surface-guided losses occur.
ZLP Elimination: By ensuring elastic scattering (Zero Loss Peak, ZLP) does not enter the spectrometer, the ZLP is suppressed by orders of magnitude, optimizing the dynamic range for low-loss measurements.
Sample Structure: Experiments were conducted on multilayered heterostructures including Nanocrystalline Diamond (NCD), GaN, AlGaN, and AlN, often prepared as thin FIB lamellae.
Acceleration Voltage Tuning: Experiments were performed at 60 keV (lower Cherenkov losses, higher momentum resolution) and 300 keV (testing robustness against high relativistic effects).

6CCVD Solutions & Capabilities

6CCVD is positioned perfectly to support research extending this advanced EELS methodology, particularly for diamond and related WBG heterostructures. Our capabilities meet and exceed the specialized material and processing requirements detailed in this study.

Applicable Materials for Replication and Extension

Research Requirement	6CCVD Recommended Material	Technical Advantage
Nanocrystalline Diamond (NCD) Film	Polycrystalline Diamond (PCD)	High-quality, reactor-grade PCD films up to 500 µm thickness, suitable for thin film heterostructure fabrication and analysis.
High-Purity Band Gap Reference	Single Crystal Diamond (SCD)	Optical grade SCD with ultra-low nitrogen content (high purity) enables precise characterization of intrinsic electronic band structure (7.4 eV direct gap) without contamination blurring. SCD available down to 0.1 µm films.
Dopant Engineering	Boron-Doped Diamond (BDD)	For extending EELS analysis to doped WBG systems or studying gap states in conductive materials, 6CCVD offers BDD with tunable doping levels.

Customization Potential for Advanced EELS Setups

The experimental rigor of off-axis EELS demands highly controlled sample and aperture preparation, areas where 6CCVD provides unparalleled expertise:

Precision Thin Films & Substrates: We supply custom SCD and PCD plates up to 125mm diameter. Crucially, our ability to provide controlled thickness films (0.1 µm to 500 µm) is vital for minimizing beam broadening and ensuring plan-parallel geometry required by the Kröger model used in EELS simulations.
Custom Aperture Metalization: The Bessel aperture was fabricated using a 1 µm thick Au film. 6CCVD offers expert, in-house metalization services, including Au, Pt, Pd, Ti, W, and Cu deposition, allowing researchers to customize or replicate specific EELS aperture geometries or reference materials with demanding metallurgies.
Surface Preparation: Maintaining high-quality interfaces for heterostructure analysis is critical. We offer state-of-the-art polishing down to Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, ensuring minimal surface scattering losses that complicate low-loss EELS data.
Laser Cutting and Shaping: 6CCVD can laser cut plates to custom dimensions needed for specific TEM/STEM holders or FIB pre-preparation steps, ensuring seamless integration into complex experimental setups.

Dedicated Engineering Support

6CCVD’s in-house team of PhD material scientists specializes in MPCVD growth parameters, crystal quality, and optimizing diamond for demanding applications. We provide comprehensive material selection and customization consultation for projects focused on:

WBG semiconductor characterization (e.g., Diamond, GaN, AlN).
Low-loss EELS optimization and signal interpretation.
Fabrication of custom apertures or reference samples requiring precise metal films.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.ultramic.2018.03.021

References

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